not sure how I'm supposed to help you, sadly - not having much of a clue what your band sounds like, what the chords are, or what you are capable of playing...

But here's a suggestion - get the guitarist to record the part, then try and sing a bass part to it, imagine in your head what the ultimate bass part would sound like, then go back and work it out - record what you're singing and then transcribe it.

There are hundreds of ways of approaching writing bass lines, and all the information that you can lay your hands on will help you - books on theory, transcriptions of lines by bassists that you admire, lessons - loads of stuff.